I would consider Highlighter to be more or less like the “snipping tool” that comes with some versions of Windows. You can “highlight” words, sentences, paragraphs, and even images on different blogs or webpages and share those things with your friends – similar to how the Windows snipping tools allows you to capture images and do what you wish with them.
The big difference is that if you highlight something, the publisher of the content receives credit and a link back. Thus, the originator of the content gets insight into how many people are using his content. All you have to do, as the content originator, is install a bit of code onto your blog/website. If you’re a user of the website you can create an account that stores all your highlights and allows you to go back and comment and make more notes on them.
This blog post from the Highlighter site highlights all the different aspects of the website, which became open to the public on July 21.
Have fun! 🙂